Turbo engine

ABSTRACT

The turbo engine comprises rotor blades made of an electrically conductive material with an electrically insulating surface, said rotor blades being rotatably mounted on a rotor shaft arranged in a housing. The electrically conductive material of the rotor blades is electrically connected to a reference potential. At least one measuring element is arranged in the area of the rotor blades, said measuring element being intended for measuring an electrical and/or magnetic field strength caused by charge distribution on the surface of the rotor blades.

CROSS REFERENCE TO RELATED APPLICATION

This application is the US National Stage of International ApplicationNo. PCT/DE2003/003411, filed Oct. 14, 2003 and claims the benefitthereof. The International Application claims the benefits of GermanPatent application No. 10251720.7 DE filed Nov. 6, 2002, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to a turbo engine comprising rotor bladesmade of an electrically conducting material having an electricallyinsulating surface, said blades being arranged on a rotor shaft that isrotatably mounted in a housing. The invention also relates to a turboengine comprising rotor blades arranged on a rotor shaft that isrotatably mounted in a housing, and comprising rotationally fixed guideblades made of an electrically conducting material having anelectrically insulating surface.

BACKGROUND OF THE INVENTION

Turbo engines having blade arrangements for interaction with a fluidflow flowing through the turbo engine are basically called compressorsor turbines and the like. In order to increase the efficiency of suchmachines, ever greater physical demands are placed on the materials usedin the turbo engine. In particular, in order to increase the efficiencyof a gas turbine, the temperature of a gas flow flowing into the gasturbine is raised to more than 1200° C. So as to be able to withstandthe high physical demands, in particular resulting from the temperature,the blades of the turbine are provided with a coating that withstands aparticularly high stress. The thermal barrier coating, referred to belowas TBC, of gas-turbine blades, is an example of such a coating, wheresuch a coating is applied to that surface of a blade that is exposed tothe gas flow.

This coating of the blade is subject to an aging that is dependent,among other factors, on the operating time and the other operatingconditions of the blade. After reaching a defined operating time, thecoating detaches from the blade, for example, which means that the bladeis exposed without protection to the high stress presented by the gasflow. Immediate replacement of the blade is necessary in order to avoidthis blade being destroyed and hence the turbine being damaged.

Thus in the prior art, operating times are specified for such blades,which once they have elapsed result in the turbine being dismantled andthe blades being replaced or recoated even if the blades do not yet showany signs of damage. A disadvantage in this case is that, in order toavoid failures of blades and the turbine, the maintenance intervals arechosen to be so short or frequent as to make it highly improbable that afault will occur between two maintenance intervals. It would thereforebe desirable for such a turbo engine not to need to undergo maintenanceuntil replacement or recoating of the blade is necessary, in particularin order to keep costs and downtimes low.

One option for ascertaining the damage to a coating of a blade beforethe blade itself is damaged is disclosed in U.S. Pat. No. 6,512,379. Inthe known device, gas flowing through the turbines exerts pressure onthe turbine blades resulting in elastic deformations of the blade. Inaddition, friction between gas and blades is created by the flow aroundthe blades. This results in radio frequency signals typically in the GHzrange, generated by a pressure-induced piezoelectric effect, anexpansion-induced electrostrictive effect or a friction-induced“tribo-charging” effect. The radio frequencies are detected by an RFantenna. Damage to the coating of a turbine blade results in a signalthat differs with respect to the other blades. This indicates damage tothe surface coating.

SUMMARY OF THE INVENTION

The object of the present invention is to use a simple other option toascertain the damage to a coating of a blade before the blade itself isdamaged.

The solution proposed by the invention is that the electricallyconducting material of the rotor blades is electrically connected to areference potential, and the turbo engine comprises at least onemeasuring element, arranged in the region of the rotor blades, formeasuring an electric and/or magnetic field strength set up by a chargedistribution on the surface of the rotor blades.

Use is made of the effect that charges tend to “settle” on insulators,whereas they flow away on conducting materials. This is exactly the casefor the TBC ceramic coating, on which charged particles from the ionizedgas flow settle, while they flow away on uncoated metal. The quantity ofcharge on the coating is proportional to the TBC surface area, resultingin a measure of the integrity of the ceramic coating. If the rotorblades now rotate past a fixed antenna, it is possible to detect thecharge differences, these being the differences from the region withoutblades (intermediate area) or between the individual blades, whichprovides information on defects in the TBC coating. A coaxial dipoleantenna that projects into the gas flow can be used, for example, forthe detection of the electric field generated by the charges (capacitivecoupling), the frequency of said field (e.g. 4800 Hz) being obtained bymultiplying rotational speed (e.g. 60 Hz) by number of blades (e.g. 80).The detection is therefore performed in the low frequency range.Information on the integrity of the blades can be obtained in the timedomain from the amplitude height in the frame of e.g. 80 cycles (numberof blades), or in the frequency domain from the appearance of harmonicsof the rotational speed of e.g. 60 Hz. Under ideal conditions, in thetime domain all amplitudes of the frame would be equal, or in thefrequency domain it would not be possible to see any sub-harmonics belowthe 4800 Hz of the example.

By means of the invention it is possible for the first time to monitorthe state of the blades, in particular continuously even duringoperation of the turbo engine, and to carry out a timely maintenanceand/or overhaul of the turbine, in particular the blades, when adefinable wear threshold is reached. As described above, the inventionmakes use of the effect that charges tend to collect on insulators,whereas on conducting materials they are conducted away by thesematerials. Charges occur in the flow of fluid materials either when thefluid itself is disassociated into relevant charge carriers, or whenrelevant charge carriers have been supplied to the fluid. The chargeaccumulated on an insulating layer may be proportional to its surfacearea, but is substantially dependent on the surface area. The chargecarriers accumulated on the rotating rotor blades generate an electricfield whose field strength can be detected by the measuring element. Anindication of the field strength can thus be obtained by an analysis ofthe signals from the measuring element. An intact blade generates in themeasuring element a corresponding characteristic amplitude of themeasurement signal (e.g. electrical voltage). A worn coating on a rotorblade or a defective coating results in the charge being conducted awayvia the electrically conducting rotor blade to a reference potentialthat is electrically connected to the rotor blade. Such a blade cantherefore generate a low-amplitude measurement signal in the measuringelement. With rotation of the rotor shaft, all the blades of a bladewheel can be directed sequentially past the measuring element, so thatit is possible to determine completely the state of the rotor blades ofthis rotor-blade wheel.

In a further embodiment it is proposed that the turbo engine comprisesrotationally fixed guide blades made of an electrically conductingmaterial having an electrically insulating surface, the electricallyconducting material of the guide blades being electrically connected toa reference potential, and at least one measuring element being providedon the rotor shaft in the region of the guide blades, for measuring anelectric and/or magnetic field strength set up by a charge distributionon the surface of the guide blades. Thus by exploiting the same effect,it is also possible to monitor an insulating surface provided on theguide blades. This further embodiment can also be combined with theaforementioned measures relating to ascertaining damage to the coatedrotor blades.

It is also proposed that the measuring element is formed by a coaxialantenna. The coaxial antenna can advantageously have a very compactdesign and consequently be integrated easily in an existing structure.Furthermore, the coaxial antenna has favorable measuring propertieswhereby distortions or measurement errors can be kept low owing to themeasurement principle used. Furthermore, using simple means, a coaxialantenna can be designed to withstand the high physical demands presentat the intended measuring point. In principle, of course, othermeasuring elements can also be used, such as electrometers and the like.In addition, however, measuring elements can also be provided thatgenerate a corresponding signal from the magnetic field generated by themovement of the charge. Thus the measuring element can also be designedas a measuring coil which can be used to detect changes in magneticfield, from which the state of the coating of the blades can bedetermined.

It is further proposed that the measuring element can be connected to ameasuring unit. The measurement result supplied by the measuring elementcan advantageously be conditioned for further processing by means of themeasuring unit.

Furthermore it is proposed that the measuring unit comprises amonitoring unit. The monitoring unit can be used, for example, todetermine when a definable threshold value is reached, with maintenanceof the turbo engine being planned when this threshold value is reached.

It is further proposed that the measuring unit has a communication linkto a control center. This not only enables permanent transmission of ameasurement result to the control center, for example in order topredict when maintenance is due or even to be able to initiate measuressuch as maintenance or activate spare units depending on the currentstate of wear, but also transmission of a threshold value being reached.The communication link can be implemented via radio, Internet or thelike for example. The transmitted data can be saved in the controlcenter for subsequent further processing.

In addition, it is proposed that the monitoring unit comprises asignaling and/or an alarm device. When a threshold value is reached, awarning can thus be issued to the operating staff so that appropriatemeasures can be taken in good time. A signaling device can be formed bya monitor, for example, on which the current measured values can bedisplayed. The values can also be displayed graphically on the monitor,however, and also compared with adjustable threshold values. When athreshold value is reached, an alarm device such as a warning lamp,flashing alarm lamp, a warning horn or the like can be actuated. Asignal can also be sent to the control center however.

It is also proposed that the turbo engine can be shut down by means ofthe monitoring unit. This can advantageously result in the shutdown ofthe machine when defects on the blades are found, before the machine isdamaged. Downtime, repair costs and repair time can be reduced.

In a further embodiment of the present invention, it is proposed thatthe turbo engine is a turbine, in particular a gas turbine. Particularlyadvantageously, it is possible to monitor continuously for damage to thecoating of gas-turbine blades subject to particularly high stresses.This saves the need for specifying fixed maintenance intervals.Furthermore, the time at which maintenance is performed can be chosenaccording to need depending on the actual state of the blade. Downtimesand costs for premature maintenance can be reduced further. In addition,by saving and processing the measurement data appropriately, the qualityof the blades used can advantageously be monitored. With a drop inquality, indicated by shorter maintenance intervals, suitable control ofthe blade machining process can be initiated.

The invention also proposes a method for determining damage to anelectrically insulating surface of at least one rotor blade in a turboengine, said blade being made of an electrically conducting material andarranged on a rotor shaft that is rotatably mounted in a housing,wherein an electric and/or magnetic field strength set up by a chargedistribution on the surface of the rotor blades is measured by means ofa measuring element, and a deviation from a definable threshold value isdetermined. Advantageously, it is possible to monitor the state of thesurface of a blade and ascertain when a definable wear threshold isreached. A plurality of measuring elements can also be provided,however, whose measurement values are compared in parallel withcorrespondingly definable threshold values. Thus, for example, aredundancy in the measuring elements can be provided so that highreliability of the measurement can be achieved. This is particularlyadvantageous for large gas turbines used for supplying power, for whicherroneous measurements would result in high costs. The state of a guideblade can also be determined, however, when a suitable measuring elementis provided on the rotor shaft. The measurement method can be appliedanalogously.

The analogous method for determining damage to an electricallyinsulating surface of at least one guide blade in a turbo engine, saidblade being made of an electrically conducting material and rotationallyfixed in a housing, thus provides that an electric and/or magnetic fieldstrength set up by a charge distribution on the surface of the at leastone guide blade is measured by means of at least one measuring elementarranged on the rotor shaft in the region of the guide blades, and adeviation from a definable threshold value is determined.

In addition, it is proposed that the deviation is transmitted to acontrol center. Advantageously a plurality of turbo engines can bemonitored by one common control center so that the monitoring overheadcan be reduced overall.

In order to be able advantageously to take an appropriate measure whenthe threshold value is exceeded, it is proposed that an alarm is outputwhen the threshold value is exceeded. This can be performed by a controlcenter, for example, although the measuring unit itself can alsogenerate and output an alarm. The alarm can be output, for example on asuitable display unit that issues a visual or acoustic signal. Thesignal can also be issued by the control center, however, if the turboengine is operated without technical staff in normal operation forexample.

In addition, it is proposed that the turbo engine is shut down when thethreshold value is exceeded. Thus a protective function canadvantageously be achieved which can prevent a damaged surface leadingto damage to the whole rotor blade. Even this measure can be performedby a control center for example. Suitable control devices which can beused to control the operation of the turbo engine can be provided on theturbo engine for this purpose.

In a further embodiment it is proposed that the measurement signalsupplied by the measuring element is transformed, in particular by aFourier transformation, by means of a measuring unit. The measurementsignal supplied by the measuring element contains a frequency componentgenerated by the movement of the discrete blades past the measuringelement. Deviations from the normal value can be determined more clearlyby the transformation of the measurement signal.

It is proposed that an FFT transformation unit is used for the purpose.The FFT transformation unit can be provided in the measuring unit, forexample, or even in a remote control center. The measurement signal canbe transformed continuously into a corresponding transformed signal bythe FFT transformation unit. Continuous monitoring of the transformedsignal can be achieved.

In order to determine the state of the blades, it is also proposed thatthe result of the transformation is displayed and signaled. Thus, forexample, the result of the transformation can be communicated to amember of the operating staff in order to inform him of the state of theblades.

In order to generate a signal or to generate a criterion for theoperation of the turbo engine, for example, it is proposed that theresult of the transformation is compared with a definable thresholdvalue. Particularly sensitive and accurate detection of the wear stateof a blade can advantageously be achieved in this way. Even a slightreduction, which can be considered as a wear indicator, can be detected,so that appropriate measures can be taken in good time, such asreplacing the blade or repairing the coating.

The threshold value can also be varied according to the blade or coatingused, however, in order to be able to take into account differentproperties of the coating or a different load. Thus, for example, thethreshold value for a blade at the fluid intake of the turbo engine canbe set at a different level, for example, than for a blade at the fluidoutlet of the turbo engine. In addition, the threshold value can also bedefined as a function of other operating parameters of the turbo engine.Thus the threshold value can be set to a higher level when the turboengine is operating under high load than when the turbo engine is underlow load.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features are given in the following descriptionof exemplary embodiments. Substantially identical components are labeledwith the same reference numerals. In addition, where features andfunctions are identical, reference should be made to the description ofthe exemplary embodiment in FIG. 1.

FIG. 1 shows a perspective view of a gas turbine whose housing has beencut open along its length.

FIG. 2 shows a side view of a coated rotor blade of the gas turbine inFIG. 1,

FIG. 3 shows a side view of a coated guide blade of the gas turbine inFIG. 1,

FIG. 4 shows an enlarged view of the turbine and compressor region ofthe turbine in FIG. 1 having a measuring element according to theinvention,

FIG. 5 shows a basic circuit diagram illustrating an operating procedurefor the gas turbine shown in FIG. 1,

FIG. 6 shows a block diagram of a measuring device for measuring themeasurement signal from the measuring element,

FIG. 7 shows a diagram displaying a Fourier transformation of themeasurement signal measured using the measurement setup in FIG. 6 forthe gas turbine running at 100% load.

FIG. 8 shows a diagram as in FIG. 7 with a transformed measurementsignal for the turbine running at 30% of the rated load.

FIG. 9 shows a diagram as in FIG. 7 with a transformed measurementsignal when the gas turbine is shut down when running at 100% load.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a perspective view of a gas turbine 1 of the prior artcomprising rotor blades 4 arranged on a rotor shaft 3 that is rotatablymounted in a housing 2 and comprising rotationally fixed guide blades 7.An air intake 18 is provided at one axial end, and a compressor 19arranged axially after it. The compressor 19 is followed by a combustionchamber 20 having burners 21, the turbine area 22 with the gas outlet 23being connected to this chamber. FIG. 4 shows an enlarged view of theturbine area 22. In FIG. 4, electric charges present on the surface ofthe rotor blades 4 are labeled with 24.

FIG. 2 shows an individual rotor blade 4 for arrangement on the rotorshaft 3, made of an electrically conducting material, preferably a metalsuch as steel or the like. The surface 5 of the rotor blade 4 isprovided with an electrically insulating coating, here a ceramiccoating.

FIG. 3 shows a corresponding guide blade 7, also made of an electricallyconducting material, which also has a ceramic coating on its surface 8.

The rotor blades 4 are grounded via the rotor shaft 3 and a bearing (notshown in greater detail) of this rotor shaft 3. The guide blades 7 arelikewise grounded via the housing 2 of the turbine 1.

During operation, hot process gas flows from the combustion chamber 20through the turbine area 22 to the outlet 23. Owing to its hightemperature of 1200° C., the gas flow contains ionized particles thattend to settle on the insulating surfaces. As a result, the surfaces 5,8 of the blades 4, 7 become positively charged. As shown in FIG. 4, acoax antenna 6 is provided on the housing 2 opposite to the first row ofrotor blades containing the blades 4. During rotation of the rotor shaft3, this coax antenna detects the field changes of the electric fieldcaused by the charge carriers located on the rotor blades 4. The coaxantenna 6 generates, in time with the charges moving past, correspondingelectric signals that are transmitted via a cable 25 to a measuring unit10. The gas turbine 1 used in this example is designed for a speed of3600 revolutions per minute, and has 80 rotor blades 4 arranged radiallyon the rotor-blade wheel located opposite to the coax antenna 6. Thus atthe design speed, 4800 pulses per second are generated, corresponding toa frequency of 4800 Hz.

The measurement procedure is shown schematically in FIG. 5. FIG. 5 showsa section of the rotor shaft 3 of the turbine 1 comprising a rotor blade4 arranged on it and a guide blade 7 fixed on the housing 2. The coaxantenna 6 measures the local electric field and transmits a signalcorresponding to the measurement value via the cable 25 to the measuringunit 10. The measuring unit 10 conditions the transmitted signals andsends them to a monitoring unit 11 contained in the measuring unit 10.The monitoring unit 11 compares the level of the conditioned signalswith a definable threshold value 15, and when the signal level fallsbelow this value transmits an appropriate alarm to a warning horn 14 andalso sends a signal via a radio link 13 to a control center 12. Thecontrol center 12 comprises a receive unit 26, which receives andconditions the signals transmitted via the radio link 13. The receivedsignals undergo a Fourier transformation in an FFT unit 16, e.g.Mathcad, and are displayed on a display 27. The display 27 can be formedby a monitor screen, for example, or even by rows of LEDs mounted in anenclosure. The display 27 also has adjustable threshold values 17 whichcan be used to indicate when the transformed signal level falls belowthreshold. A monitoring unit 28 continuously compares the transformedsignal level with the threshold values, and when the signal level fallsbelow a threshold value, transmits an appropriate signal to a controlunit 29 of the turbine 1. The turbine 1 can be shut down via the controlunit 29. Thus if wear is ascertained, the turbine 1 can be shut down andmaintenance initiated.

A gas flow coaxial with rotor shaft 3 contains ions owing to itstemperature of about 1200° C. The positive charge carriers of the gasflow deposit themselves on the insulating surfaces 5 of the blades 4.The charge carriers of the blades 4, which are carried past the coaxantenna 6 by the rotation of the rotor shaft 3, create correspondingmeasurement signals that are processed according to the method. Ifdamage to the insulating surface occurs on a blade 4, for examplebecause of wear, the charge located on the surface 5 of the blade 4decreases by being conducted away to ground at least partially via themetal body of the blade 4. The reduced amount of charge leads to acorresponding reduction in signal from the coax antenna 6, whereby theaforesaid measures are initiated automatically when the signal fallsbelow a defined threshold value 15.

FIG. 5 also shows a corresponding monitoring for a guide blade 7,connected to the housing 2 of the turbine 1, said blade likewise havingan insulating surface 8. The guide blades 7 are also correspondinglypositively charged on their surface 8. A corresponding measuring element9, designed as an induction sensor in this embodiment, is provided onthe opposite shaft section of the rotor shaft 3. The induction sensor 9rotates at the axial height of the guide-blade arrangement and measuresin this way a magnetic field generated by the differential motion. Thecorresponding signal is transmitted via connections (not shown ingreater detail) to a measuring unit designed for this purpose, which canhave a communication link to the measuring unit 10 so as also to have aneffect on the system of the gas turbine 1. In principle, however, acapacitive measuring element such as the coax antenna 6, for example,can also be provided at this point.

FIG. 6 shows a measurement setup for measuring the signals supplied bythe coax antenna 6. The coax antenna 6 is connected to an amplifier 30for this measurement, the output signal of which amplifier drives theinput of a transient recorder 31, the time signal of which istransformed by an FFT in a PC.

FIG. 7 shows a signal-level-frequency diagram of a data record, saved bythe transient recorder and transformed in Mathcad, from the gas turbine1 of FIG. 1 when operating under 100% load. The diagram has a Cartesiancoordinate system whose ordinate gives the relative power level of themeasured signal, while its abscissa is the frequency in Hertz. A singlepeak at the frequency of 4800 Hz, corresponding to the pulse sequencespecified above, is clearly visible.

A diagram as in FIG. 7 is shown in FIG. 8, where the power equals just30% of the rated power of the turbine 1. A peak at 4800 Hz is againquite clearly visible in this case.

FIG. 9 shows a diagram as in FIG. 7, but with the turbine 1 run downfrom 100% load to the idle state. Even in this case the peak at 4800 Hzis clearly visible. It can be seen here, however, that the ordinatevalue of the peak depends on the power of the turbine. Thus in order toachieve a practical monitoring of the coating, the threshold value 17 iscorrected according to the current power state of the gas turbine 1.Thus the effectiveness of the coating on the surface 5, 8 of a blade 4,7 can be found for every power state of the gas turbine 1 irrespectiveof the operating state of the gas turbine 1.

The exemplary embodiments shown in the figures serve merely to explainthe invention and do not narrow its scope. Thus in particular the typeof the measuring element or the further signal processing and thearrangement of the measuring element and also the number of themeasuring elements used can vary without going outside the scope ofprotection of the invention. In particular, dual elements can obviouslybe used, for example a measuring element for a magnetic field instead ofa measuring element for an electric field, because it involves themeasurement of charges moved relative to the measuring element. Inparticular, the monitoring of the coating of a guide blade by means of ameasuring element rotating with the rotor shaft is included. A pluralityof measuring elements can advantageously be arranged in succession inthe direction of the rotor axis. It is preferably possible for eachmeasuring element to be assigned to a ring of turbine blades. It isthereby possible to determine on which ring precisely the damage to thesurface coating has occurred. In addition it is possible using asynchronization pulse, for example correlated with the line frequency(e.g. 60 Hz), to determine on which blade precisely the damage to thesurface coating has occurred.

1-20. (canceled)
 21. A turbo engine, comprising: a plurality of rotorblades made of an electrically conducting material having anelectrically insulating surface and arranged on a rotor shaft that isrotatably mounted in a housing and electrically connected to a referencepotential or a plurality of fixed guide vanes made of an electricallyconducting material having an electrically insulating surface with theelectrically conducting material of the guide vanes electricallyconnected to the reference potential; and a measuring element formeasuring an electric and/or magnetic field strength set up by a chargedistribution on the surface of the rotor blades or guide vanes, whereinthe measuring element is arranged near the rotor blades and/or near theguide vanes.
 22. The turbine engine as claimed in claim 21, wherein atleast one measuring element is arranged on the rotor shaft in the regionof the guide vanes.
 23. The turbo engine as claimed in claim 22, whereinat least one measuring element is arranged in the region of the rotorblades and at least one measuring element is provided for measuring anelectric and/or magnetic field strength set up by a charge distributionon the surface of the rotor blades.
 24. The turbo engine as claimed inclaim 23, wherein at least one measuring element is formed by a coaxialantenna.
 25. The turbo engine as claimed in claim 23, wherein at leastone measuring element is connected to a measuring unit.
 26. The turboengine as claimed in claim 25, wherein the measuring unit contains amonitoring unit.
 27. The turbo engine as claimed in claim 25, whereinthe measuring unit has a communication link to a control center.
 28. Theturbo engine as claimed in claim 26, wherein the monitoring unitcomprises a signaling and/or an alarm device.
 29. The turbo engine asclaimed claim 26, wherein the turbo engine is shut down by themonitoring unit.
 30. The turbo engine as claimed in claim 23, whereinthe electrically insulating surface is formed by a coating.
 31. Theturbo engine as claimed in claim 23, wherein the turbo engine is a gasturbine.
 32. A method for determining damage to an electricallyinsulating surface of a turbine component, comprising: providing aplurality of turbine blades or vanes made of an electrically conductingmaterial and arranged within a turbo engine; creating an electric and/ormagnetic field strength by a charge distribution on the surface of theturbine blade or vane; measuring the electric and/or magnetic fieldstrength by a measuring element; and determining a deviation from adefinable threshold value.
 33. The method as claimed in claim 32,wherein the measuring element is arranged on a rotor shaft in the regionof the vanes.
 34. The method as claimed in claim 32, wherein thedeviation is transmitted to a control center.
 35. The method as claimedin claim 32, wherein an alarm is output when the definable thresholdvalue is exceeded.
 36. The method as claimed in claim 32, wherein theturbo engine is shut down when the definable threshold value isexceeded.
 37. The method as claimed in claim 32, wherein a measurementsignal supplied by the at least one measuring element is transformed bya Fourier transformation, by a measuring unit.
 38. The method as claimedin claim 37, wherein a FFT transformation unit is used.
 39. The methodas claimed in claim 38, wherein a result of the transformation isdisplayed and/or signaled.
 40. The method as claimed in claim 39,wherein the result of the transformation is compared with the definablethreshold value.